Australian cucurbit production

Vegetables account for approximately 7% of agricultural production in Australia, and was valued at $4 billion in 2018–19 (ABARES 2019a). The contribution of cucurbit commodities is valued at around $520 million annually across the country (Horticulture Innovation 2020). The Cucurbitaceae family encompass a number of popular domesticated species, cultivated for their fruit and seeds, including zucchini and squash (Cucurbita pepo), pumpkin (C. moschata, C. maxima), watermelon (Citrullus lanatus), rockmelon (Cucumis melo) and cucumber (Cucumis sativus) (Grumet et al. 2021). Cucurbits are produced commercially in all states and territories of Australia that experience a predominant tropical, subtropical or temperate climate, with the exception of the Australian Capital Territory and Tasmania where the cool climates are unsuitable (Grumet et al. 2021). A smaller number of producers grow more bespoke species including wax gourd (Benincasa hispana), gourd (Lagenaria siceraria), luffa (Luffa spp.) and bitter melon (Momordica charantia). Cucurbita moschata and L. siceraria are also produced as rootstock varieties which are grafted onto other cucurbits such as watermelon as a disease management strategy for the soilborne fungus Fusarium oxysporum f. sp. niveum and F. oxysporum f.sp. cucumerinum respectively (Crino et al. 2007). Within Australia, zucchini, squash, cucumber and pumpkin production falls under the broader banner of the vegetable industry and they are typically grown almost year-round in the temperate coastal zones. Melons (rockmelon, honeydew, watermelons) are grown as summer cash crops in the warmer southern and central west parts of NSW and Queensland, and also in the dry season in Western Australian and the Northern Territory. Commercial growers often grow multiple cucurbit commodities on the same property.

Cucurbits are a common summer crop (temperature or sub-tropical zones) or dry season crop (tropical zones) for many growers and are seasonally impacted by a broad range of fungal, bacterial and viral pathogens, with the focus for this review on the latter. Viral disease is a persistent problem with frequent outbreaks during the summer growing period, particularly for field grown crops, often leading to high production losses. Globally, more than 60 viruses have been documented impacting cucurbit production, with new species and strains being continuously identified both through the advent of more sensitive diagnostic options and increasing surveillance efforts (Lecoq and Desbiez 2012). Comparatively, only a few key viral pathogens account for the majority of viral related cucurbit crop losses in Australia and their effective management is a key concern for the vegetable industry (Table 1) (Maina et al. 2017a, b; Coutts and Jones 2005; Dombrovsky et al. 2017). In-field diagnosis of these cucurbit viruses is problematic as many exhibit common symptoms or can present as asymptomatic in some hosts (Lecoq and Desbiez 2012). Crops, both field and greenhouse, can be simultaneously affected by multiple viruses resulting in more pronounced symptoms (Wang et al. 2002) and reports of crops being 100% affected by virus during the peak of the growing season are not uncommon (Persley 2012).

Table 1 Cucurbit viruses endemic to Australia
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Preparedness

Preparedness activities for exotic viruses require a systematic approach and a significant planning phase, with support and adoption by producers, peak industry bodies, government biosecurity and research firms as a cooperative effort (Carnegie and Nahrung 2019; Craik et al. 2017). A significant body of work is required to understand the epidemiology of viral pathogens, the susceptible host range, inoculum threshold required to permit infection, and the abiotic factors that permit establishment and spread. It also allows for strategic decisions to be made for incursions of new viruses, related to the containment and eradication options available (Arya 2018). Rigorous surveillance and risk assessment allows for potential transmission pathways to be identified and, where necessary, border security strengthened or market access carefully managed to mitigate the introduction of new viral strains into Australia (Craik et al. 2017). Furthermore, it creates a critical time window for diagnosticians to become familiar with new symptoms and ensure diagnostic platforms are maintained or improved to incorporate new pathogens at short notice when required.

Maintaining a watchful eye on virus issues beyond our borders is an important component to any viral management strategy for Australian cucurbit producers as we remain free of many of the pathogens causing economic impact in other countries (Pheloung 2003). Key objectives include measures to minimize the entry of exotic diseases and mitigate the establishment and spread of new pathogens (Beale et al. 2008). Trade with affected areas must be carefully monitored and appropriate risk assessments made to reduce the chance of new viruses entering and becoming established within Australia, particularly for seed borne infections. Once key threats have been identified internationally, the determination of the pest status within the Australian biosecurity system is a critical step (Anderson et al. 2017). This should be developed in concert with appropriate assessment of import conditions, industry biosecurity plans, contingency plans, and diagnostic capacity. The existence of these measures prior to an incursion allows biosecurity agencies, research teams and industry to operate from a place of strength during a biosecurity emergency should an exotic virus be detected (Plant Health Australia 2007). Consequently, a lack of adequate preparation would significantly hamper the success of an attempted eradication program and potentially contribute significant financial cost to industry were the pathogen to become established. Significant time lags in detection may also prolong the time required to achieve successful eradication of the pathogen, leading to greater financial costs, or compromise the ability to achieve eradication at all.

The increasing globalization of the seed trade heightens the risk of importing new seed-borne viral pathogens and the rate of geographic expansion (as evidenced by new detections) of many viral pathogen groups appears to be accelerating over the past two decades (Dombrovsky and Smith 2017; Constable et al. 2021). Australia remains vulnerable to new seed-borne infections being introduced as virtually all commercial cucurbit seed is imported. Although Australia maintains a rigorous pre- and post-border biosecurity system incorporating inspections and testing of plant material (Craik et al. 2017), it is not practically feasible to screen for all known pathogens. Instead, risk assessments determine which pathogens should be awarded the highest priority for screening. Additionally, it is worth noting that not all seed may pass through the appropriate import pathways and hence bypass the required pathogen screening tests (Constable et al. 2021). Whilst not a common practice for commercial entities, there has been an increase in the online purchase of seed by home gardeners (Pheloung 2003; Constable et al. 2021). The receipt of unsolicited seed products as seen in the recent, well-publicized “brushing” scam that saw seeds from China sent to several countries to artificially inflate sales, including Australia and USA is also of concern (DAWE 2021). Seed of this nature poses a significant biosecurity risk to Australian plant industries as it is uncertified, typically of unknown provenance, and diagnostic screening has detected a number of high priority quarantine pathogens (Constable et al. 2021).

Surveillance and diagnostic screening

Due to the large number of samples and viruses required to be tested in a given consignment, it is not practical to test every sample for every known virus. Strategic surveillance programs should explore versatile approaches such as metagenomic or HTS (high-throughput sequencing) pipelines to clarify the presence of any cryptic viral pathogens, beyond the standard diagnostic screening panels. This option, however, may present its own challenges in terms of characterizing new disease detections, and the triggering of a biosecurity response, if new pathogens are uncovered (MacDiarmid et al. 2013). Distinguishing pathogenic viruses from a ream of sequencing data also requires careful consideration to determine the viability and epidemiology of newly identified candidates (Thomas et al. 2017).

High throughput sequencing and more sensitive molecular diagnostics are helping to refine viruses in a more detailed way than ever before (Rodoni 2009). The validation of available assays for molecular diagnostics is required to ensure the assay is suitable and reliable for use within an Australian context. This may require greater international collaboration, to provide positive control material for the validation step, to ensure cross-border preparedness (Rodoni and Geering 2007). Assay validation may facilitate the refinement of existing diagnostic assays to tailor detection in an Australian context to ensure endemic Australian viral isolates are able to be clearly distinguished from exotic populations. International vigilance and collaboration with Australian agencies is encouraged as it allows for new pathogens to be incorporated into standard surveillance and diagnostic operations. Targeted screening can then verify that these pathogens do not already exist within Australia and maximize detection possibilities at border interception points.

Further surveillance is required within Australia to better identify resident but previously undescribed pathogen populations (MacDiarmid et al. 2013; Carnegie and Nahrung 2019). It is feasible that there are endemic viral populations that are yet to be mapped in Australia. These cryptic populations may pass under the standard screening process as they are either asymptomatic, have overlapping symptoms with known viral pathogens or are not routinely tested for (Coutts and Jones 2005). Conventional viral surveillance techniques may be supported by new technologies such as in-field diagnostic platforms, remote sensing and sophisticated camera technology linked with drones and digital detection software for detecting infected crops (Thomas et al. 2017). Previous studies have suggested plant viral pathogens may have migrated from our close geographic neighbours in South-East Asia. The monsoon season is of particular concern where viruliferous insect vectors may be blown between countries (Maina et al. 2017b; Davis et al. 2021; Sastry 2013). Similar programs need to be regularly implemented in other southern cropping areas to capture pathogens that may have entered the country via human mediated or natural pathways. Routine surveillance even of endemic viral populations is also required as it is likely that existing viruses will evolve over time and will need to be characterized and better understood as they are detected, especially if that genetic divergence impacts virulence or host range (Coutts and Jones 2005).

Beyond scrutinizing the pathogen population, emerging vegetable industries within Australia need to be carefully managed, particularly for biosecurity risk associated with the importation of new seed or planting material. It is critical that this material has been adequately screened for exotic pathogens to mitigate the introduction of new pathogens. New crop species or varieties being introduced to Australia may themselves be susceptible to endemic viruses, as is the case for the bitter melon (M. charantia) industry that is currently being developed in Australia. This is an edible subtropical cucurbit crop, but it may be susceptible to viral pathogens already present in Australia such as potyviruses or tobamoviruses (Naik et al. 2019). Instances where this has not been properly considered can result in significant economic impact to growers, as in the case of the introduction of cassava (Manihot esculenta) into Sub-Saharan Africa. Cassava crops were introduced into new growing regions in Africa that intersected with the natural range of cassava mosaic disease (a suite of several viral pathogens), triggering widespread viral epidemics in cassava production areas (Akinbade et al. 2010; Alabi et al. 2015). This event caused significant production losses and lead to growers abandoning cultivation of cassava entirely in some areas (MacDiarmid et al. 2013). Periodic epidemics of cassava viruses in Africa have resulted in severe economic losses (in the order of US$1–2 billion annually) and humanitarian crises caused by food shortages (Varma and Malathi 2003). The impact of viruses on cucurbit production can be substantial, but the effect on broader food production is not usually as severe as cucurbits are rarely a staple food resource.

Biosecurity risk – exotic pathogens

The Australian Federal government revised the National Priority Plant Pest (NPPP) list in 2019 to identify key exotic threats to Australian plant industries (ABARES 2019b). Pests and pathogens on this list are targeted for specific surveillance programs, development of diagnostic protocols and contingency plans as part of the ongoing biosecurity preparedness in the event of their incursion. In addition, emergence of new strains and virus transmission between close geographic neighbours, particularly in northern parts of the country, was recognised as a key biosecurity issue for Australia (Maina et al. 2017a, b). The number of pathogens cited in the NPPP list is extensive and pertain to a range of plant commodities however the primary concern for the cucurbit industry is the list of exotic tobamoviruses, currently ranked 32 of 42 on the list. Tobamoviruses belong to the Family Virgaviridae and are composed of a positive strand RNA virus genome (Lecoq and Desbiez 2012). These viruses can be seedborne and mechanically transmitted, inflict significant damage on commercial crops and pose a significant biosecurity threat to Australian vegetable industries (Dombrovsky et al. 2017). Although 13 viruses are mentioned specifically in the NPPP list, the viruses listed in Table 2 are pertinent to cucurbit production. Additional exotic viruses that pose a threat to Australia’s cucurbit production have also been referenced in Table 2. These viruses have been recognized as an emerging issue internationally in vegetable production, have recorded a host range within the Cucurbitaceae family however do not currently appear in the NPPP list.

Table 2 Exotic viruses of concern to the Australian Vegetable Industry
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Exotic begomoviruses and their insect vectors are featured in the NPPP list, and ranked 40 of 42, although specific viruses have not been listed. Begomoviruses belong to the Geminiviridae Family and have a single stranded mono- or bi-partite DNA genome (King et al. 2012). Of all the geminiviruses, begomoviruses perhaps pose the biggest threat to cucurbit production as they are transmitted persistently by whitefly Bemisia tabaci complex (Rosen et al. 2015). Additionally, the vector population is expanding geographically, and so too is the range of these viruses (Varma and Malathi 2003). The broad diversity of geminiviruses and recombination events are contributing to the increasing complexity of this group which may present additional future threats to cucurbit production as new pathogens emerge (Varma and Malathi 2003).

Exotic tospoviruses, Family Tospoviridae genus Orthotospovirus are another key threat to the Australian cucurbit industry identified in the Biosecurity Plan for the Vegetable Industry (Plant Health Australia 2019). Tospoviruses are persistently transmitted by various thrips species (Persley et al. 2006; Riley et al. 2011; LaTora et al. 2022; Nagata et al. 2004). These viruses also pose a threat to the broader vegetable industry, with infections recorded in the plant families Solanaceae, Fabaceae, and Amaranthaceae (Kenyon et al. 2014; Holkar et al. 2017; CABI 2022; DAWE 2017). Orthotospoviruses are not known to be seed transmitted but could be introduced into Australia on imports of fresh produce including cut flowers and fruit and vegetables or via infected thrips vectors (DAWE 2017). Watermelon bud necrosis virus (WBNV) and groundnut bud necrosis virus (GBNV) are specifically mentioned in the pest risk analysis for their potential impact on cucurbits (DAWE 2017). These viruses are not known to be present in Australia but are widely reported as having a significant impact on melon production owing to the necrotic or chlorotic lesions induced on leaves, buds and fruit (Plant Health Australia 2019; Singh and Krishnareddy 1995; Holkar et al. 2017).

In addition to industry specific threat assessments and contingency plans mentioned in this review, the European and Mediterranean Plant Protection Organization (EPPO) has recognized watermelon silver mottle virus (WSMoV) as an emerging threat to cucurbits internationally. WSMoV is a tospovirus that has inflicted significant leaf and fruit deformities leading to yield reductions in melons in Japan, Taiwan and China (Yeh and Chang 1995; Rao et al. 2011; Mou et al. 2021). WSMoV induces silver mottle in melons, leaf deformities, stunting and yield decline (Rao et al. 2011). Melon yellow spot virus (MYSV) should also be considered as a key biosecurity threat given its increasing prominence within Asia (Kato et al. 2000; Peng et al. 2011; Okuda et al. 2002) and the presence of the thrips vector already within Australia (Layland et al. 1994). These viruses are not known to be present in Australia (CABI 2017, 2020).

Globally, new viral strains are continuously being identified as surveillance presses into new areas and molecular diagnostics become more effective. Intense selection pressures and high biological variability is possibly driving the surge in new virus detections along with the continued natural mutation, recombination and diversification of existing viruses. Potyviruses are presenting new viral threats to cucurbit production as surveillance efforts expand into new regions. Recently, several emerging strains of the WMV pathogen were identified in Europe, rapidly becoming the dominant strain in lieu of the classical strains of the virus in production regions (Desbiez et al. 2009). Initially, papaya ringspot virus (PRSV) was considered to be a distinct virus but is now considered to be part of a “cluster” of up to five viruses. Originally, Moroccan watermelon mosaic virus (MWMV) was thought to be a new strain of watermelon mosaic virus (WMV) but has since been revealed to be a distinct potyvirus following more detailed molecular investigation (McKearn et al. 1993) and more closely aligned with the PRSV cluster (Yakoubi et al. 2008a). Algerian watermelon mosaic virus (AWMV), isolated from squash in Algeria may also share a common ancestor with MWMV (Yakoubi et al. 2008b; Ibaba et al. 2016). Two new viruses, tentatively named wild melon vein banding virus (WMVBV) and Sudan watermelon mosaic virus (SuWMV) have also been characterized as part of the PRSV cluster (Desbiez et al. 2017). These pathogens share a high genetic, biological and serological similarity but retain enough divergence to be recognized as individual viral species (Lecoq et al. 2001; Desbiez et al. 2009).

There can be a high degree of biological variability within each virus species. There may be subtle differences in pathogenicity, host range and even disease expression depending on the source of the isolate (Desbiez and Lecoq 1997). Initially, zucchini tigre mosaic virus (ZTMV) was considered to be the T-strain of the PRSV cluster and induced a “tiger stripe” pattern on affected zucchini (Romay et al. 2014). More recent sequencing analysis has revealed that ZTMV is distinct from PRSV, and it may be more closely related to squash chlorosis mosaic virus (SqCMV), if not the same species (Abdalla and Ali 2018). Often, ZTMV is often found in mixed infection with other potyviruses such as PRSV and is slowly progressing through South American production zones (Romay et al. 2014). Other emerging potyviruses include zucchini yellow fleck virus (ZYFV), which is being increasingly detected in the Mediterranean region, France and Israel (Tomassoli et al. 2010). Also, zucchini shoestring virus (ZSSV) was reported in Africa (Ibaba et al. 2016). As new virus species or disease complexes are recognized it is important that epidemiological changes such as host range or transmission method are understood to facilitate better management and improve the chances of detection.

Symptom expression in host crops is affected by variety, timing of infection and environmental conditions. Some infection generates asymptomatic plants and further complicates in-field detection (Dombrovsky et al. 2017; Simmons et al. 2011). Often, symptom expression in alternative plant hosts is attributed to other causes such as nutrient deficiency and these hosts may not express symptoms at all, further hindering virus detection (Tessitori et al. 2002). Field surveys in America targeting cucurbit viruses have observed that as many as 80% of infected plants were asymptomatic and would otherwise have been overlooked in a visual inspection (Prendeville et al. 2012). Moreover, the introduction of crop species into new geographic regions is increasingly placing susceptible host species at the intersection of established virus territories (Jones 2009). It is likely that further pathogenic strains will be identified in time as the geographic ranges of native and agricultural plant species further intersect (Gibbs et al. 2008). Therefore, new diagnostic platforms will be required as new viruses or strains are identified. Already, surveillance work conducted across Northern and Western Australia has identified several native plant and weed species exhibiting symptoms of potyvirus infection that did not produce any positive results to currently available virus diagnostic assays (Coutts and Jones 2005).

The rapid appearance of cucurbit viruses globally over the last 40 years was facilitated on several fronts. Human activity has been a key component, as international markets expand and seed, germplasm and plant material are exported to an increasing network of countries (Gibbs et al. 2008). Incomplete or non-existent diagnostic screening protocols for seed and transplants, changes in quarantine restrictions and the existence of latent or asymptomatic infection has also aided virus establishment in new regions (Jones 2009; Boonham et al. 2014). As the geographic range expands, unfamiliarity with new viral infections, and asymptomatic infections, can result in a time lag before virus identification, potentially allowing new infections to circulate and establish unnoticed (Rodoni 2009). Genetic mutations and recombination events may see new viral strains being able to proliferate within a wider range of host species, contributing to range expansion of the viral population (Malmstrom et al. 2011). Increasing genetic diversity coupled with advances in molecular sequencing technology will see increased detections of novel viral groups. For instance, watermelon crinkle leaf associated virus 1 and 2 (WCLaV-1 and WCLaV-2) was first detected in China in 2017 (Xin et al. 2017). Their detection generated new virus information impacting cucurbit commodities, and it led to the putative description of a novel clade in the Family Phenuiviridae (Xin et al. 2017).

The way forward

Plant viruses have evolved rapidly since the dawn of modern agriculture in response to the changing ecological situation (Jones 2009). Now, questions are being raised as to the best management approach moving forward in light of climate change, population growth, changing agricultural practices and increased pathogen/vector movement globally. This dynamic environment is already influencing the transmission and epidemiology of pathogenic viruses, both within the natural environment and in agricultural systems (Jones 2014; Malmstrom et al. 2011).

Estimates provided by the United Nations’ Food and Agriculture Organization has projected that demand for agricultural productivity will increase 60% by 2050 (Bajželj et al. 2014; Van Dijk et al. 2021), as the global human population is projected to tip 8–9 billion by the middle of the century (Cohen 2001). This will place increasing demands on food production and an even greater need to mitigate disease-related yield declines. Australian producers are in a relatively privileged position in that the viral diversity affecting cucurbit production is relatively small at present, but the threat of exotic pathogens entering Australia’s production system is ever present, particularly as market access becomes more globalized.

There is an urgent need to develop new viral management solutions for our food crops. Capitalizing on novel technologies to enhance plant defense responses or genetic means of manipulating host resistance to key viruses will play an increasingly important role in mitigating viral related crop losses. Embracing new technologies, such as smart HTS diagnostics or remote sensing to detect and map viral pathogens, could be an exciting prospect for the vegetable industry (Oerke 2020; Yang 2020; Maina et al. 2021). Whilst the application of precision agriculture in the cucurbit space is still in its infancy, the prospects for disease monitoring utilizing this technology has the potential to revolutionize disease detection in the field (Berdugo et al. 2014). The challenge for the cucurbit industry will be to drive increased focus of emerging technologies for cucurbit commodities to future proof disease management programs. Novel techniques, such as priming plant defence responses with biological products (Katiyar et al. 2014, 2015) or the application of RNAi (RNA interference) related products (Mitter et al. 2017a, b) are still applied to model host plants such as Arabidopsis thaliana. Translating these ideas to crop host models is an important validation step of new protocols and technologies.

In Australia, disease control is typically considered on a property-by-property basis rather than on a meaningful biological or geographical scale. The most suitable approach to managing viral infection may differ widely among producers due to personal preference, knowledge and expertise, past experience, cost and individual business strategies. This can result in a variety of mismatched approaches being employed in close proximity, limiting the effectiveness of any one program both within and beyond the farm gate. The identification of common goals and viable virus control strategies at a broader geographic scale would enable the adoption of an Area Wide Management (AWM) program. This approach is likely to have a greater chance at success and allow the collective benefit to pervade beyond individual property boundaries. Effective AWM programs promote ownership of the strategy by all participants, and factor in flexibility and scalability to meet changing needs over time (Spiegel et al. 2005). Developing an AWM strategy must be addressed in tandem with the needs of industry and current, scientifically validated approaches to managing viral infections, to ensure maximum uptake of any future strategy. There must be leadership within the growing community, the scientific field, industry and government to ensure AWM programs are aligned to common community, environmental and economic objectives across all tiers of society. Continuous community engagement to communicate AWM strategies is required to ensure disease mitigation strategies remain relevant and allow for adaptations to be incorporated when new management options become available or new threats are identified.

Virus management options are limited and for countries like Australia, the need to prevent the introduction of new pathogens is a high priority. Appreciation of the diverse transmission pathways is important for the adoption of relevant phytosanitary measures, stringent import conditions and appropriate crop disposal options for infected host material. It is critical that we understand the diversity of viral pathogens already present in our production landscape so that informed and timely decisions can be made regarding their management, and with regard to the incursion of new exotic species.